I don't see to many problems with handing out some potions as help as given in your example. It helps your group to recuperate a bit faster... also without upgrade these potions are useless at the end of the adventure. So no problem there. And the help that any given contact can wield is - IRRC - left quite sketchy. It might be that a trader contact offers advice and better prices, a mercenary contact might help you out to return a favor or for good coin, a goblin chieftain might offer the help of his tribe in a certain quest to repay an favor and so on.
The only thing you might possibly keep in mind is, that the NPC might need the workshop for herself. So sure sharing it might not a problem, but fulltime occupation by the player would probably abuse the relationship with the NPC. If they pay for the privilige on the other hand fulltime occupation might not be a problem. The Herbalist still needs to make income and that comes - so it sounds to me - from crafting potions of all kind.
In general I wouldn't come up with a list what is possible when and how much on the level of the relationship that an NPC and a PC have. For me such things depends how they treat their Contacts/Friends/Allies and how certain social limits/implications limit or favor certain things. The relationship between a commoner and his contacts will be different than that of a noble and his contacts.
In your example the use of the workshop might be okay, even if limited, but using the supposedly "secret" recipes out of the herbalist's book certainly wouldn't. These are after all secrets of her trade (supposedly).
Also I don't see a problem why she wouldn't help the group. Her continued survival might be in the hands of some battle-hardened adventurers who can fight back any threat. That's how I read/interpreted your post.
Try to keep in mind how you would react in certain situations or take another POV and use that to bring up a believable response or just go with your gut feeling. It can also help to write up some character traits before the game to remind you how you want to portray the NPC.
Keep in mind, that I gloss over some things to keep the game fluid at my table.
So your mileage may vary.