Mikey: Donnie is a stand up guy, I trust him with my life.
A loyal and reliable friend.
The allusion is to someone who would be prepared to stand up and fight on your side if called on, i.e. one who, in the words of the earlier (late 19th century) phrase, would 'stand up and be counted'.
The phrase is American. The earliest citation is in the Pennsylvania newspaper The Charleroi Mail, April 1935:
Standing up to questioning.
Guy: I dunno what that is or how it got there. Never seen it before.
Cop: Oh yeah? We're gonna lock up up and throw away the key if you don't tell us where you got that stuff.
Guy: I don't know NOTHIN' about it!
Dig it, you are a stand up guy!