We read 8,2 as eight point two.One of my students have written eight comma two. Can I accept it? What do you advise?

Here's a lesson on commas with numbers.

Firstly, there are two conventions:

1,001.2 (UK and US)
1.000,2 (European)

It looks like you're using the European one. Writing "eight comma two" is pretty inefficient. Unless there's a good reason for it, I'd point that out to him if I were you.