The sequence of events is this:
1. Mc gets mad, leaves Erryn's backyard, goes to what is likely his garage, and seethes.
2. Erryn turns to Ethan and asks, "Why do you always have to bully him?" Then Ethan responds, and Erryn tells him it's not funny. Alexis peaces out. That exchange takes maybe a minute.
3. While they're arguing, MC has a convo with Alexis, goes into his house, has a convo with his mom, and then goes to talk to Erryn
4. The argument ends, and MC runs into Ethan on his way back to Erryn; Ethan lies to MC, which sours MC for good.
I think what happened was Ethan bullying MC pissed off both girls. Alexis felt sorry for the MC and went to talk to him, while Erryn gave Ethan a piece of his mind and tried to stop Ethan from bullying him. I imagine Alexis didn't have to look for him; she just followed the direction he was going and saw him sitting in front of his garage.