@stephanieloves2 This is what you get wrong. I've been in the same place as op before I started bouldering and there is almost no carry over from general pulling strength to the very specific finger strength.
Like even the easiest rungs on my portable hangboard would blast OP's and other calisthenics guys fingers as hanging on a bar no matter how long may strengthen your forearm but there is almost no stimulus for the tendonds or ligaments to grow in a way that’s required in climbing or bouldering.
Your advice goes completely against what is the bottom like in climbing, which is NO MATTER how strong you think you are, climb or boulder 3 times a week for at least half a year and THEN start to carefully hangboard.
If he wants to go bouldering from next week on he should just do that to train his fingers and not, I repeat NOT start hangboarding before he even climbed, it makes on sense on the gains side as just climbing will be sufficient and he'll Risk to just end up like so many other people that blew their tendonds because they thought they’re special bEcAuSe ThEy cAn HaNg fRoM a bAr aNd Do mUsClE uPs.