Short answer... "No, it's not legal." First off, if he's forging the signature of his old boss, that's fraud right off the hop. If he has signing authority, and signs with his own name, it's a different story, but forging is fraud regardless. Furthermore, it's fraudulent to make yourself out to be something that you're not... whether it's an MVA, CRA, or AACI, his appraisal's aren't worth the paper that they're written on if he's not accredited.
Now, if he's providing a basic market analysis of just comparable sales for information purposes, he might get away with that because most banks won't accept those reports anyways. But, if he's saying that he's an actual appraiser, he's digging himself into a hole. It'll cave in on him one day.
Just a little insight into the future for your friend as well.. if a bank has been using him as an appraiser for financing purposes, and they find out that he's not an actual licensed appraiser, they'll probably hire a new appraiser for all the past appraisals that he did and your friend will probably be footing the bill for all of them.