First of all, there is ALWAYS a risk of ban, never forget that. If you are in a custom game with your friend, then I think no precautions are needed.
I'm also using this trainer even in public matches since maybe 4-5 days, and yet no ban.
Since you're in a custom game with your friend, ban chances aren't that high but it may be still possible.
Don't overuse it. And avoid having the program open in public matches.