I disagree. The broadest definition of possession is probably the federal definition. If your friend can control the guns, then he's in possession of them. For example, if I have my friend hold onto a gun for me across town, I am still controlling it, even if I can't touch it. That's possession. But just being in the vicinity of a gun is not possessing it.
If you keep the guns locked up in the safe, and your friend can't access them or tell you what to do with them, there's no problem.
It is true that you friend takes a risk by living in a residence where there are firearms. If he is actually on probation or supervision, you may want to get the arrangement cleared with the probation agent before moving in together.
My answer to your question is based on the facts that you provide in your question. Additional factual details about your situation could change my answer completely. The law in inherently uncertain and always subject to change.