Your friend is either naive or very cheeky!
Of course you should say a permit is needed if they want to leave a car on your street.
Not your problem!
Perhaps let them leave the car and let the fines pile up, how is this your problem or favour to offer?
Not as if you have a private drive .
As for permits for friends, make sure you get the online ones as cheaper.
If you don’t feel like paying, just say you have run out of them or reached your limit.
Is it resident only or pay and display? If they can pay via machine - why bother offering is it’s that frequent?
Look in the mirror and practise how to say no to people -
trust me , it will be awkward at first but feel so good when you’ve done it .