Can a lesbian can have a baby boy?

Yes. Only if she gets pregnant or adopts.
Equal chance. A persons sexual orientation does not change the gender chances of conception. The lesbian would need a male sperm donor to become pregnant. The sex chromosome of the sperm that fertilized the egg will determine the sex of the fetus.
Yes. Any woman of child bearing age can have a child, assuming she is otherwise healthy. Her sexual orientation is not associated with her fertility, rather her choice of partner in her relationships.