My mum, South Wales charges £10 a for grass.
Be aware of everything and get a contract document signed.
Like the term of the agreement, the day the money is due. Notice should rent not be paid.
If her horse gets injured by your ponies or a faulty fence/gate. If her saddle gets nicked from the shed.
If her pony goes through a fence or hurts one of yours who's liable for repairs and vet fees?
Feed can become hassle if she wants to feed differently from you - will you just split hay cost three ways? Will she take her pony out of field to feed it a bucket (if you/she do/don't bucket feed).
Worming timing, ideally done as/before she moves in and thereafter all wormed together.
Just some tips: most you'll never need to think of, then you get someone who won't pay, won't move their horse out, is hitting the other ponies with a whip so she can bucket feed her horse, and claims that you owe her £100 because her rug was nicked from the roadside gate while she went out for a ride!