romans joining eastern monasteries

[MiscarriageSucks]

my dear friend is joining a Ukrainian byzantine monastery soon, please pray for him. 

my question is can any roman catholic just join an eastern catholic monastery? to my knowledge he hasn't transferred over, but maybe he did it without telling anyone.  just dunno if being catholic is all they care about to join any eastern monastery or if they require transference of Rites. 

it's in california if that is pertinent.