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, that is, one who, in the words of the earlier (late 19th century) phrase, would 'stand up and be counted'.
The phrase is, of course, American. The earliest citation I can find for it is in the Pennsylvania newspaper The Charleroi Mail, April 1935:
"But he [Babe Ruth's employer, Jacob Ruppert] seems to be a 'stand-up guy' and loyalty, with him. seems to be less a virtue than obsession."