Does anybody have any experience with the Hmong language? My friend who is fluent tells me it would be easy to learn since the grammar is simple, but I would imagine it would be hard for westerners without any experience in tonal languages.

I agree. I think tone and other phonemic features of Hmong not present in English would more than compensate for an apparent 'simplicity' of grammar, making it quite tricky to gain fluency in the spoken language. By some counts, Cantonese has seven tones - like Hmong, I believe. Nouns don't decline, are invariable for number and there is no grammatical gender; verbs are not conjugated and the tense system is very basic compared to English, etc. etc. but many non-Chinese who attempt to learn the language struggle at even the most basic level to make themselves understood.

Not specific to Hmong, but if it's similar to other tonal languages that I have some familiarity with than I would say that you could get to an advanced beginner level / intermediate rather quickly (mostly because of very simple grammar). To get to a truly fluent level it will probably take longer than other western languages.

Hmong language is tonal.
Depending how you finish the last sound, the word could have a totally different meaning.
Hmong have more than 7 tones, but Hmong is as easy as Cantonese or Korean.
Hmong writing system is romanised and very easy to read and understand.
kuv yog hmoob = "gooh yoh Hmong"
Do a google for Nerida Jarkey in Australia. She has dedicated part of her life to the studies of Hmong language.
I am one of her research assistants. And yes, I am Hmong in Australia.