The hostility is part of the spell description.
And this is very balanced. This is a cantrip that can be cast once every round as many times as you want. If it were not balanced with a negative response by the target it would be very easy to overuse it. The Friends cantrip description is balanced in two ways:
- It ensures that the target knows who cast the spell when the spell ends.
- It provides a negative consequence for using the spell. That is the hostility of the target when the spell ends.
Both of these points are clearly illustrated in the spell description as shown by the highlighted text (PHB 244, emphasis mine):
When the spell ends, the creature realizes that you used magic to influence its mood and becomes hostile toward you.
The roleplay is not specified
But it is indicated in a general way. The target becomes hostile. What does this mean? In standard usage for the spell it probably would mean that the target is angry that their feelings were manipulated, or something along those lines, and the target acts in an angry way towards the spell caster. However in this usage it could mean something else.
For example, even when a person knows that their mood has been influenced they are often in a state of emotional and even physical discomfort when an "enhanced mood" wears off. Think of caffeine or nicotine withdrawal. You know you had caffeine, you know it made you feel more awake and "perky", you know you would not have made it through the work day without it. It's 5pm, its effects are gone, and you're annoyed.
How are you annoyed? Do you have a headache? Are you fidgety? Are you exhausted? Do you yawn every ten seconds? What are you annoyed at? The caffeine? The coffee? The work day? Yourself? Traffic? Everything? How do you respond to this annoyance? Do you drink more coffee? Drink some calming tea? Speed in your car? Yell at people? Listen to music?
GM decides the roleplay
There are many ways to feel and act hostile towards a person. In the situation you describe, where the target is knowingly affected by the spell to enhance a performance there are many possibilities:
- The target could be irritable and want more of the spell to continue to feel good.
- The target could want to experience the performance again (right now!).
- The target could feel that the enhancement wasn't all it was cracked up to be and wants their money back.
- The target could be on a downer after such an amazing "enhanced" performance and be generally angry at everything around them.
- The target could be acutely aware that this was the best performance they will ever see and no performance will ever match it because of the enhancement you provided - you essentially just ruined every future performance they will ever see, and they know it. This makes them very angry.
- The target could demand that you accompany them next Saturday night to a different performance that you're not even a part of.
These are just some possibilities.
How the target is roleplayed is entirely up to the GM and there is a vast spectrum of possibilities.