Okay, in my plot there is a girl dying of Lukemia... What is the probability of a boy who works at the hospital having a marrow match?

Partially, it seems to depend on what type of leukemia she has. (Additional information here, here, and here. And remember, Google is your friend.)

Here's some info on bone marrow transplants. The National Marrow Donor Program has LOTS of info, and this page and this page list some highly contrasting numbers that might help figure out the odds. (I'm no good with numbers, but I'd say they'd be astronomically against that scenario happening, from what I can tell.)

Hope this helps you!
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One issue you'll have to work around is if he is truly a boy (i.e. underage), he wouldn't be tested for a match--unless there was a compelling reason to believe he might match and his parents agree. Situations where this happens for people underage are 1) family members--cousins, etc. and 2) members of ethnic minorities that have a very hard time finding matches in the existing donor pool.

You could also think about having a desperate time of things--no match possibilities anywhere, girl is dying, the whole town has a "get tested" drive for EVERYONE to participate in, to try to find a match--and the boy's whole family goes and gets tested. That sort of thing.

It is very unlikely that he would be a match unless they are related... somewhat close relation... and/or are of a genetic minority. Is the "boy" someone who works at the hospital?

The boy is a kid who is failing school and needs to do summer school. So, he is sent to work at the hospital, where he meets the girl.

How old is he though? 17 or older?

held back a year, so should be a senior but is a junior