On the issue of muscle soreness, my instructor told me to make sure I eat two bananas a day and a glass of skim milk to make sure I'm getting enough potassium in my diet. If you work out seriously, you have to eat right both to have the proper energy for the work outs, and to recover well from them.
There's wisdom to what both of you are saying here though. If you're going to do a move, then yeah, you should try your damndest to do it right. But doing it right means keeping proper form and proper balance of muscle tension for whatever particular move you're doing. Often times I've had to do some workout routines I thought were pretty brutal, and when I couldn't keep proper form for some of the things I had to, my instructor showed me alternate moves of lower difficulty that would train the muscle groups the original move was designed to. It's a hell of a lot better than getting injured.
On the other hand, people often underestimate what their actual limits are, and wimp out way before they should. There's a lot to be said about learning to push yourself when your brain is begging you to stop. If you've just started working out, take a bit of time to ease yourself into a routine, either working out fewer days (3 days a week with the actual routine, stretching and maybe lighter workout on a 4th day) or lowering the intensity of the workout you do initially. But if what you're taking is called "Beginner's Yoga" or something similar, I recommend the first option, and to push yourself like hell on the days you do workout. If you're not out of shape to the point of being handicapped, and the program developer knows what she's doing, then the intensity level should be properly gauged. In which case, push yourself and use this time to learn what those limits actually are.