There's not enough info in your post to tell you for sure, but it could be that the tub drain isn't in a usable location for a shower.
If your father-in-law is a retired pro and has seen the bathroom first hand, I'd have to defer to his assessment. His rationale for mentioning the toilet is that the toilet is probably right next to a waste/vent stack, and the shower drain would need to connect to it.
Also AFAIK in most jurisdictions you would require a 2" drain for the shower, and you may have something smaller for the tub right now.
Chances are you'd have to modify your tile job anyhow when you do the shower conversion (if not for plumbing reasons then for aesthetics), so you might want to do them at the same time.
If your father-in-law is a retired pro and has seen the bathroom first hand, I'd have to defer to his assessment. His rationale for mentioning the toilet is that the toilet is probably right next to a waste/vent stack, and the shower drain would need to connect to it.
Also AFAIK in most jurisdictions you would require a 2" drain for the shower, and you may have something smaller for the tub right now.
Chances are you'd have to modify your tile job anyhow when you do the shower conversion (if not for plumbing reasons then for aesthetics), so you might want to do them at the same time.