Well if you want to do more than just say sorry... keep it small.
Just send her a gift card to Lowe's or get a prepaid VISA gift card or a nice clean fresh bank bill of cash that can be used wherever and tell her in a "I'm sorry" card that the next plant is on you. (Don't use a nasty wrinkly one if cash -- just get a crispy one from you bank, makes the card so much nicer)
There. You made tried to make ammends with grace. Next time just do not agree to something you are not comfy doing. You don't have to be people pleaser if it lands you in places you are not comfy in. That's all the baggage YOU have to hold and tend to.
She wants to make a fusspot after that, that's HER baggage, so let her hold her OWN bags, hold her own tempest in a teapot. You don't have to drink her tea.
Her mad, her upset that she pushied a known plant killer into tending and you killed the plants, etc. That's not your fault, it's her bags to hold, she took the risk. She's gotta own it. You were not deceitful in your plant killer ways. (joking to make you feel better)
There's subirrigation you know -- I have subirrigation planters that go 1-2 weeks without my having to top up. She could have repotted in one of those and topped it before she left. She could have taken it home to the house sitter, whatever on her travels. Don't blow it up bigger than what it is. Just give her space to get over herself.
Not end of world. Stop worrying.
A.