Soapmaker here who does wedding favours. Evidently some people do like them, because I never did them until people started to approach me and ask for them.
My opinion is it depends completely upon the type of soap and also the strength and attractiveness of the branding. If she's skilled and is doing proper cold-process, and has all of the relevant legal paperwork, then it could be great. I have to say I would be worried that she's willing to do them for free - the set-up for being a professional soapmaker is expensive, when you take the safety assessments into account, and the ingredients for good soap are likewise expensive. This is why good-quality handmade soap costs the price it does. You get what you pay for, unfortunately. Full-sized bars of really good soap would work out really expensive if you have 70 or so to buy.
There are soapmakers using melt-and-pour bases - this isn't regarded by most of us as 'real' soapmaking, but it is much cheaper. The product is less kind to the skin, as it needs to be formulated in a certain way to allow repeat re-melting. There isn't a lot of skill involved, unless the soapmaker is doing really elaborate decorations on them. It still needs to have the relevant paperwork.
Regarding the scent - the people saying it will make the room smell horrible are, I think, used to Lush-type scents which are very far from natural, extremely powerful in terms of scent throw, and can be problematic for many people with sensitivities to artificial scent. Soap scented using essential oils give a scent which projects much less and don't seem to give a problem when they are spread throughout the room, particularly if the soaps are small (together in the box, they do smell stronger). I don't use artificial scents myself and certainly wouldn't want it at a wedding. They are cheaper to use, though.
As a branding exercise I'm not convinced it will work. I'm not aware whether I've ever gained a customer because somebody saw my soap at a wedding - I think it's usually the other way around, in that people who know my soap and can vouch for the quality then opt to use it at their wedding.
In my case, people ask me to do the favours because I'm a known local/handmade ethical small business with an attractive branding that is strongly tied to the local area. I wouldn't ever expect to get an order for wedding favours from outside the area. The soaps I make for weddings are not full-sized, which would cost too much for favours I think, but mini hand-soaps of the type you'd use in a downstairs loo.
I put the couple's names on the wrapper, not on the soap itself. I also provide full ingredients listings and an allergen declaration in case any of the guests needs to know this. I don't know whether anyone has ever asked to see it, though.