This is, quite simply, horrible advice. If you are strength training regularly and slowly increasing the weights you are using, mild muscle soreness should be

This is, quite simply, horrible advice. If you are strength training regularly and slowly increasing the weights you are using, mild muscle soreness should be expected. If the soreness is bothering them, there are several options that are known to help - stretching, additional warmup or cool down exercises, ice, heat, or massage. The signs that could suggest a protein deficiency are decreasing muscle mass, weakness, and/or swelling, but even then you would suggest a trip to the doctor, not simply to increase protein intake.