If I am in your shoes as a second photographer/amateur/backup/whatever photographer, here's what I will do.
Let your friend know clearly that you serve as a backup photographer to capture shots. And tell your friend who agreed to your shooting to tell the hired photographer of his decision. Greet the hired photographer on first meeting and explain that you do not intend to disrupt his/her work. Like the chinese saying goes, "Dou Dou Zhi Jiao". You are not there as an adversary to the main photographer. You are there for the fun of photography. Tell your wedding couple friend that you bring value to them as you will bring another perspective to the shoots, despite being an amateurish attempt or an advanced amateur effort.
As you are the 2nd cameraman, it is important to practise some photographer etiquette and not obstruct the main photographer or videographer, if any. A photographer and a videographer are already crowding the area. as the saying goes.. two's a company, three's a crowd. A 2nd photographer will crowd out the angle further. So stay clear if the space is tight and concentrate on the less-focused activity.
Bear in mind that the main photographer is paid to produce the goods. So do not catch an angle at the expense of blocking the main photographer. They are making a living or at least paid to produce. So do not disrupt their work, or at least minimise any chance of that happening.
Once you showed you respect the main photographer and not obstruct their work, I am sure the main photographer will have no problem with a second "amateur" photographer doing the candids. And maybe even give u a tip or two.
I learnt a lot from a real pro that way.
on the other hand, a main photographer may find that you are co-operative enough and ask you to co-operate on some shots later.... apart from learning from his experience and knowledge.
my 2cents