question about 2nd shooting but not official to the pro...

my friend from these shots https://photography-on-the.net/forum/showthre​ad.php?t=396661

asked me to take pictures on their wedding day...
I'm not the official pro, because they already have a pro...
They want me to just take some of the more casual pictures of the guest and details...
I told them to ask the pro first if it is ok with them...
I'm not sure if they've asked or not... but their wedding is coming up soon...

my question is, if in this situation, should I shoot with or without flash?
I don't want to bother or ruin the pro's shots?

thanks

Talk to the pro at the start of the day, ask what they'd prefer. Generally you should be shooting different things from the pro anyway, not copying his shots, so it shouldn't much matter. Only time it might really matter is if he's doing a long exposure that your flash manages to go off during, which seems unlikely.

Why not contact the pro yourself?

I rarely go to a wedding where there is not a guest with an SLR - usually there's a few of them floating around. As long as you don't get in the pro's way, I think you'll be all set. And yes, bring a flash (but try not to use it during the ceremony).

you should ask the pro on the start of the day. and i don't think he will mind and he will respect that you even minded bothering him...
as jamiewexler said, a lot of guests with SLRs might ruin things up.
In Egypt, we have guests with Point & Shoot camers and mobile phones cameras that know how to really ruin your pictures and sometimes not even give you a place to stand for your own shots

I went to my cousin's wedding and he already had a pro, I politely ask the pro if I can shot and ask him to let me know if I got into his way (by accident).
He even let me try his 1D Mk3 during the buffet time !

I shot a wedding on Saturday and there were around 200 guests and I'd say about 30-40% had cameras and always managed to get in front of me when I had them lined up for a shot! I think that cameras are just too affordable these days