stemdamage?

So my buddy's got some plants that I set him up with as seedlings but is now having this happen to most of his plants if you guys need more photos for a diagnosis im sure he'll send me more i feel bad like its my fault even though mine are showing no signs of this help me save my buddies plants

Any one know what this could be. Only on this one plant and doesn't feel soft or rotten. Plants are about 5-6 weeks old. Side note have done some lst(Bent main stem down a little bit) Thanks from my first grow. Edit: There is no yellowing of leaves/plant looks great. About 81 degrees in the...