Can a father with ab positive and a mother with ab negative have a b positive baby?

Yes. Ab blood type you have one gene for a and a gene for b. If both parents give the b gene the child will be b type blood. Rh + can be one or both genes rh+ or one - and one +. Rh- you both genes are rh-. So if baby got rh+ form one parent and rh- from the other the baby would be rh+.