I have a client who used Lao MN, a Mac font in a document that they need adjustments made to. Does anyone know the closest Windows equivalent? Thanks for the help in advance.

That looks like an extended Times New Roman turned into a semi serif. When the font comes with a recent Mac OS it is probably open type and should also be usable on a Windows machine.
The same Latin characters can be found in quite a lot of Asian characters fonts like Khmer MN and so: http://www.dafont.com/fr/forum/read/39728/khmer-mn