I am getting quotes to convert our attached 2 car garage to a living area for my recently widowed mother; this would give her a bedroom/sitting room and storage - the attached wall follows down my interior hallway and I've asked the home improvement guy to quote me a door at the end of hall to new living area to share the existing half bath - but I need a shower added.

He has indicated that since my home sits on a concrete slab that "adding" a shower would require drainage that just isn't there, so adding drainage would be a huge cost to incur (he's talking over 10K! )

Within 2 foot radius of the existing toilet is the bathroom sink plus the washing machine which has drainage.

Isn't there a way to use the washer drain as long as you don't shower and do laundry at same time?

Not that big a deal springz.
Yes it would involve taking up the floor from the lavatory drain line, to the shower trap and raiser. The shower would be wet vented by the lavatory drain and you could take the hot and cold right off the lavatory supplies.
10K sounds like it's too pricey for a shower install. Get three estimates and consuder doing the labor yourself, such as jackhammering up the cement and digging in the trench. This would save a lot of $. You decision! Good luck, Tom