I want to call a friend that lives in Mexico, just don't get what prefixes to add apart from ++5255.
Her signature states this:
Nextel ID. 62*21XXXX*2 / 36246XXX

I've replace some numbers with X's intentionally but the number of characters are there.

Sounds like your friend is in Mexico City. Dial 011 for international long distance then 52 for the Mexico country code and then the 10 digit phone number of your friend.

you dail the area code and the whole number.

They changed the rules recently in Mexico to dial to a mobile phone. I am not sure if they are considering the Nextels as a regular cell phone.

To dial a "regular" cell in Mexico city you would dial:
011-52-1-55-xxxx-xxxx

if they are not considering Nextel's as regular Ceil phones you would dial only:
011-52-55-xxxx-xxxx

Since you are using the 55 in your question I am assuming your friend lives in Mexico city. 55 is the area code of Mexico city and the phones in there have 8 digits.

God luck.

Based on the ID you sent I would try:

011-52-55-36246XXX
or
011-52-1-55-36246XXX