What you need to do before you talk to her is, write down every chore you have done in the last week-10 days. Track the hours you have spent doing them and figure out what she'd have had to pay a minimum wage barn hand. I don't know where you are, but be aware that some states have "Agriculture Labor" laws that are much lower than minimum wage pay. You need to figure out if you are doing more or less than $250/mo worth of extra labor. You also need to take the value of your 30 mins lessons into account. The average cost for a 1 hr private lesson around here is between $65 & $80/hr, so 1/2 hour would cost $35 or $40. Deduct that from what your laboring for. Also make a list of ALL of the lessons you've received, dates and length of the lessons, so that you can show you're being fair about crediting her with having given the lessons.
Then go find another place to house your horse in case things don't go well. Then go talk to your friend and see what she says. Keep everything businesslike and don't get all emotional or drag your friendship into it, that needs to stay out of the arrangement.