Is it illegal to ask for compensation from a business associate in exchange for recommending her business to another?

Generally, unless there are professional restrictions (like lawyers for instance) You can put it in a contract. Be very careful about your relatioship with your current employer and the elderly person. There are guidlines about undue influence and lack of cpapacity. You are definitely in the grey area.

To add to the previous post, if you (or better, your attorney) determine that there are no ethical/statutory/regulatory rules against the type of arrangement you describe, referral and intriductions for compensation can and often are memorialized in contracts. You need only look to the finance and entertainment worlds for common use of such contracts that require payment to a party that made a referral to a party that invested in the business venture of the contracting party.

I hope this helps.

Disclaimer: This answer is for informational purposes only and does not constitute general or specific legal advice, nor create an attorney client relationship.