If a provider I have seen multiple times suggests bringing one of her friends along, what is expected in terms of the gift?

...Some want the sum of their usual hourly rate. Some will do a double at a slightly lower hourly rate for each. If the time is more than an hour they may want some percentage of their combined rates. Sometimes when one provider is having a "friend" join in for a double it is to help the friend get started in the business so they offer a special rate. Bottom line this is a discussion to have with the provider suggesting the double. Be sure to have the rate set and agreed upon before making the appointment. Don't go in leaving it to be worked out at the time of the session.