There's a whole variety of options.
There are actual language schools, like the one out of Koch's guest house and another from the El Colibri hotel, and even a third one on some side street of El Batey.
I can't say how good any of them are, nor if they are worth the money.
Milka takes English lessons from the local colegio in El Batey, and I bet that it's 1/10 the price of the private language schools.
If I were you, I would ask "the novia" to investigate the options at the public schools and go from there.