1. You should explain to them that when it comes to websites, you cannot achieve 100% consistency across all the zillions of variables, you can only take whatever steps are within your power to achieve as much consistency as possible. The way DA01 puts it in their answer is a very succinct and easy way to get the message across.

2. I would then tell them to decide upon font-style/s first rather than a specific font.

3. Then, only after deciding on the style, look at the options available with a systematic approach from worst possible conditions to the ideal conditions.
   Start with basic system fonts (the 5 universally supported fonts) and work outwards. They could choose a few system fonts that are less widely supported and then a single ideal webfont. (You don't want to load and render lots of webfonts on one page as you probably know).

Don't approach talking about the task as if it is open to discussion, instead explain to them that it is their responsibility as a good designer to know and account for these things if they want to successfully design for the web.

There is another recent question that may be useful for you:

Is it acceptable to use different but similar typefaces for print content and web content?

1. You should explain to them that when it comes to websites, you cannot achieve 100% consistency across all the zillions of variables, you can only take whatever steps are within your power to achieve as much consistency as possible. The way DA01 puts it in their answer is a very succinct and easy way to get the message across.

2. I would then tell them to decide upon font-style/s first rather than a specific font.

3. Then, only after deciding on the style, look at the options available with a systematic approach from worst possible conditions to the ideal conditions.
   Start with basic system fonts (the 5 universally supported fonts) and work outwards. They could choose a few system fonts that are less widely supported and then a single ideal webfont. (You don't want to load and render lots of webfonts on one page as you probably know).

Don't approach talking about the task as if it is open to discussion, instead explain to them that it is their responsibility as a good designer to know and account for these things if they want to successfully design for the web.

There is another recent question that may be useful for you:

Is it acceptable to use different but similar typefaces for print content and web content?

As suggested by another answer, pointing them to webfont services is a good course of action.

Another option is to explain to them that when it comes to websites, you cannot achieve 100% consistency across all the zillions of variables, you can only take whatever steps are within your power to achieve as much consistency as possible.

I would then tell them to decide upon a font-style rather than a specific font, whilst looking at the options available on whichever webfont website you decide is most appropriate for this project.

After that, you could (depending on their level of technical involvement), ask them to deliver a font stack (a list of fonts in order of which one you want most to which one you want least), with the first choice as their chosen webfont, and then as many fallback system fonts that come native with Mac and Windows as they can find and accept.

Do not approach talking about the task as if it is open to discussion, because it really isn't. As I explained before, you cannot achieve 100% consistency with websites.

Another option is to tell them to design everything with the default Arial and Helvetica, to be sure that it looks good without any special fonts, and then choose the webfont and fallbacks after everything else is prepared. The choice of font probably shouldn't affect other choices such as the structure or other design elements, unless the text elements are strong focal points.

There is another recent question that may be useful for you: Is it acceptable to use different but similar typefaces for print content and web content?

1. You should explain to them that when it comes to websites, you cannot achieve 100% consistency across all the zillions of variables, you can only take whatever steps are within your power to achieve as much consistency as possible. The way DA01 puts it in their answer is a very succinct and easy way to get the message across.

2. I would then tell them to decide upon font-style/s first rather than a specific font.

3. Then, only after deciding on the style, look at the options available with a systematic approach from worst possible conditions to the ideal conditions.
   Start with basic system fonts (the 5 universally supported fonts) and work outwards. They could choose a few system fonts that are less widely supported and then a single ideal webfont. (You don't want to load and render lots of webfonts on one page as you probably know).

Don't approach talking about the task as if it is open to discussion, instead explain to them that it is their responsibility as a good designer to know and account for these things if they want to successfully design for the web.

There is another recent question that may be useful for you:

Is it acceptable to use different but similar typefaces for print content and web content?