The forces experienced by your friend will be the same as if he was free falling at 160 knots and opened his parachute. The fact he was in the plane when the parachute deployed makes no difference, because in both cases he is slowed by the parachute from a high speed relative to the air to whatever the speed of a parachute descent is. The rate of slowing will be determined by how much drag the deploying parachute creates.
According to http://en.wikipedia.org/wiki/Parachuting the forces experienced when a parachute opens are 3-4G. However there are two differences to your friend's situation. Firstly free fall is usually about 120mph and you friend was travelling at 160 knots, so the drag on the parachute, and therefore the decelleration your friend experienced will be higher. Secondly it was your friend's reserve parachute, not the main one, that opened. I don't know how the reserve chute differs from the main one, but it's entirely plausible the drag is different and I'd guess lower.
Because of the variables involved I don't think we can more than guess at the forces experienced by your friend, but the 3-4G of a normal parachute drop is a good starting point. I'm not surprised that your friend's colleagues described him as "instantly gone". I've seen a friend accelerate downwards at 1g when caving (he was on a safety line!) and my recollection is that one moment he was there and the next he was gone. A 4G deceleration would indeed seem virtually instant.