Is there a free HTML5 + CSS template for a website to display a basic company info and a software product?
Yes, there are many, a search for "website template" will bring you to groups and collections of templates, some free some paid.
I recommend bootstrap templates. Bootstrap is a framework of predefined styles that are forward compatible and mobile ready.
Many of the bootstrap templates come with default content and images so you just have to replace them with your own.
Just about all of the bootstrap templates are parralax ( vertically scrolling) and so it's easy to add or remove sections to trim it down to a simple product demonstration site.
It would be good to get a little more info for this, for example: what sort of product is it? What are the companies goals with the site? Is eCommerce needed?
But in the mean time, take a look at: https://themeforest.net/
Loads of good, flexible templates on there that are easily controlled by Wordpress.
I'd recommend you do a little customising to match your clients branding though, even if it's just the colours or font, it can go a long way in making the company seem more legit.