Snapsafe Treklite. I had one of the steel lockboxes but they’re heavy as hell if you’re carrying one bag. This one’s only 2lbs. Would obviously be no match for a thief and/or some tools but it’s perfectly good for little hands, air travel, etc.
Have you spoken to a therapist about this? Keep in mind that therapy is best for regular maintenance rather than after an emergency has already happened.
Long term:
Get the guns out of the house or get out of the house. Ask your friend how he's feeling and what he wants to do with his firearms.
Short term:
Tell the owner of the house that you don't feel comfortable living with unsecured firearms, which is a very legitimate feeling for anyone in any situation and don't let anyone tell you differently (imo fetishistic gun owners are the cancer killing gun ownership for the responsible gun owners). While it may be difficult to have some sort of accident with a gun the consequences are catastrophic, additionally, guns are a significant target for burglary because they have a high black market value; so they should always be kept secure, especially in a roommate situation when random people may be in and out of the house.
If they are handguns, you can get small and very robust metal lockboxes for them relatively cheaply. Cable locks that prevent the slide from closing are even cheaper and often free from your local police department (at least here in the South they are). If they are rifles/shotguns, you can get trigger locks that prevent the trigger from being accessed or pulled. Then give the keys (usually just two per lock) to your two roommates.
No lock is as secure as the guns not being in the house, but it will make them impossible to use without tools and significant effort to defeat the locks.
I use this- very affordable and fits both wife and my pistols, together.
SnapSafe Treklite Lock Box - XL, Key Lock https://www.amazon.com/dp/B06XRD61CC/ref=cm_sw_r_cp_apa_6OQKBb53PQTSH
Snapsafe Treklite Lock Box with Key Lock, X-Large 75212, Portable Polycarbonate Handgun Safe & Case, TSA & CA DOJ Approved, Measures 10” x 7” x 2” https://www.amazon.com/dp/B06XRD61CC/ref=cm_sw_r_cp_api_lxxmAbTEDGDJH
I use this one and put it in my checked bag. It’s lighter than the steel ones and supposedly uses the same material as football helmets and such so it’s strong and doesn’t add much to weight in my bag. And then I put a TSA lock on the outside of my bag. I haven’t had any problems so far.
Just be aware it’s small but I got my Glock 19 and 2 mags and a box of 50 rounds using S&B packaging (the nice small box).