One dear to me is trying to do this quest at a level 12. We can not find a way for him to retreat.

How to get back outside please, once you are in the basemant?

thanks

save the game before entering strange places. when i do that place, i have good gear and chems. not sure if you can exit the way you came in.

Well thank you.

Yes, save often and in different slots so you can reload from an earlier save if needed. That said, you can exit the museum the same way you entered.