A payment plan? Why would you agree to such a thing? Your tenants already have a payment plan: they pay rent in full on the first of the month. You NEVER agree to take partial rents, you just don't do it. When you do it the first time, you guarantee it will never be the last time, not just with that tenant but all the rest of them, too.
The fact that you did not respond to her request is irrelevant. In absence of you actually agreeing to her request, her usual payment schedule is still in effect. If anything, it's more helpful that you didn't respond to it at all; responding to it gives her the opportunity to argue about it.
Let her think all she wants about filing anything against you. Rule #1 when it comes to people threatening to sue: 99% of them never even look into actually doing it, they are just all talk. Even if they are not, there is nothing to address or worry about until if/when you are served with legal notice of a suit. So just flush all that gossip and talk out of your head.
Do NOT agree to a payment plan. If she wants to argue that you are discriminating against her, let her; the burden of proof is on her, not you. Do NOT agree to a payment plan, if you do you will have nothing but more problems.
Do NOT agree to a payment plan for any tenant. The agreement is and always has been that they pay rent in full on the first. If they cannot fulfill their end of the agreement, then they get a 3 day notice and it progresses from there. Nothing personal, it's just business.