OP is also friends with the hostess, so I think she can maybe mention it if they are talking about the planning of the party. But it was already stated by the owner of the house that there are some restrictions (I have never been to a baby shower with silly string!) and mom is forking money over to help, so there are 3 people who need to sit down and come to an agreement.
And, I'm sorry, but I will stand by my point of, if you are throwing a party for someone, be it a shower, birthday, anniversary ect, the GOH's tastes and likes and dislikes need to be taken into consideration. I have been to some appalling showers that were so tacky (every game at one bridal shower was filled with sexual innuendo and there were kids in attendance!) and the GOH seemed so disappointed because it was not anything like them..it was all what the host wanted. Yes, the host's are spending the money, but if they are offering to throw this party to celebrate the GOH, it should be something the GOH should not have to endure.