I would dump her & she can keep it if it means that much to her.
I had something simular the other month, a long time custy too. She asked me to do the roof on her con as an extra. I told her a price, not a problem but when i was done she questioned it. :

She thought if she waved a cheque in my face what she paid 18 months i would take it, NO WAY

. I would rather nothing than getting told what i'm getting paid. The best bit she thought i would still clean her windows.
While i was on the round this month she had a trad guy turn up to quote, 4 windows there is no way he could do. When he came out he came over to say hi, i had to clue him up what happened to me

, even better he went & knocked on her door & turned her down, i was well happy.
Macc