I will offer some general income tax advice here. Without knowing your specific tax situation…this should not be construed as tax advice.
Generally, you should be claiming any revenue for the purpose of income tax. You can use reasonable** repeat reasonable expenses to reduce the amount of net income on that revenue. In the case of a leased horse, I would at least include any equine insurance you carry for the TWH and routine vet bills. If you file this on a schedule C, you may qualify for the 20% qualified business income deduction. This gives you a 20% deduction on your business income.
Now…the IRS hates financing hobbies. If you get any repeat any use of the horse you need to prorate any expenses. Say your lease says you have use of the horse 3.5 days a week, you can only use 50% of the equine insurance expense against your lease income. In your instance, I would avoid expenses like non-routine vet bills, expensive farrier or bodywork, or your board bill. That is just screaming for an IRS audit. If you were leasing a high dollar show hunter? I would consider some of those expenses…but not all.
Regarding Venmo: one of the recent tax law changes (I have forgotten which one) required that companies such as Venmo, Paypal and Zelle send you (and the IRS!) a 1099K if your annual transactions are over $600 (formerly it was $200K or 20K transactions, I think). This means that Venmo WOULD be reporting your lease income. In recent weeks, the IRS has backed off of the $600/1099K requirement for this year. It was going to create a lot of headaches for people who were reimbursed for lunches, had a yard sale, etc. Keep an eye out on that if she continues to use Venmo in 2022.
this is not personal tax advice
ETA: @#$% it’s 2023 now! Keep an eye on legislation for 2023 RE: Venmo.