I'm not sure I agree.
My son has played pairs for most of his cricket career as a wicketkeeper/batsman. He has had plenty of games ruined (for him) with his precious 12 balls being slung all over the place and being unhittable. However the bowlers are protected by a rule that limits the number of additional balls that the bowler can deliver (1 from memory). So he stands there as his 14 balls roll along the floor or are 8 feet wide as his opportunity to contribute and have fun goes up in smoke. Your spinner has been hurt by her own lack of skill- the batsman gets punished for the bowlers lack of skill.
A couple of further points specific to your example-
Is 17 really classed as that young to need extra protection? I'm not convinced.
Aren't all spinners at risk of being expensive when things don't work properly? I would think the mental resilience needed to cope with that was an important skill for all spinners to learn, and captains to manage.
Tonight I watched a 12 year old walk off the pitch in tears having got out 4 times in his 4 over partnership. He's not very good. The rule designed to protect him (loss of runs, keep batting) actually led to repeated failures rather than a single low score. In truth, no rule is going to protect him here- he failed and was going to feel bad. If he can't learn to accept that, I can't see him continuing to play the game for long irrespective of any accommodations made for him.
The point I'm trying to make (whilst probably sounding completely heartless
is you cannot legislate a poor player against a good one. If this spinner is being hit to the boundary repeatedly then it sounds like she's up against a batsman that is better than her (at least on the day) - in the same way my son's team mate was against bowlers better than him. My son has played games in all sports where he has been heavily beaten- and dished out heavy defeats. It's part of playing sport.