I would definitely recommend you to also learn her language. This way you go through a similar process and will be able to find similarities and differences in grammar and roots of words and you can motivate each other.
As for grammar rules you could use websites like https://thedutchonlineacademy.com/ that have explanations in both Dutch and English. Here you can find information about word order https://thedutchonlineacademy.com/en/category/word-order
You could also invest in a book (examples are De Opmaat or De Sprong) to combine grammar with reading and listening exercises and to have a very structured learning path.