Not sure if your question is more for predicting ride quality, or if this is a possible/worthwhile project. As far as ride quality is concerned, I can’t say, but wider tires at a lower pressure goes a long way.
Regarding if this will work, short answer is maybe, but it could get pricey. To keep it simple, keep the mtn rims as there really isn’t a lot to gain by going with the 650c tires. Your LBS may not have a lot of street mtn tires, but they are available online.
I like the cantilever brakes as road brake levers should work with them. V-brakes are a different story, although there is one version of V-brakes that are supposed to work with road levers, but if you go this route be sure they are compatible.
For the bars, be sure the stem is the same size as the bars. Most mtn stems are for a 26” bar. These can be found in the drop bar version.
Shifters can be an issue as the mtn front shifter usually will not work with a road derailleur. The back derailleur of a mtn bike and road shifters will work. When I did this conversion I chose the bar-con shifters. Works well as the front is friction based.
The two biggest questions are fit and cost. Not sure how the fit will be for her without any measurements. Just remember that the reach is extended a bit when you go to the drop bars as the brake levers are moved forward from where they would normally be on a mtn bar.
Add up all the costs first:
Tires, tubes, bars, stem, brakes(?), bar tape, cables. Then think about the gearing as you may want to change out the cassette and even the front chainrings.
Chances are you could drop a bit of coin on a bike that is neither a mtn bike nor a road bike. I mentioned I did this for myself. I don’t regret it, but I am still going to be getting a road bike and keep the frankenbike as a commuter.