They say that you never know anyone's marriage.
Beth was passive aggressive, and Buddy hid behind her skirt:
1. Mavis gets a baby announcement? Clearly a jealous, knife-twisting Beth there.
2. Buddy says that Beth doesn't drink, but then Beth is drinking? He lied, and she did drink: "Pump and dump."
3. Beth sets Buddy and Mavis up to be alone on the night of the concert. He kisses her, but Mavis ends up being talked about behind her back by the baby-naming party-goers. Clearly, Buddy ratted Mavis out to look like he wasn't over-involved.
4. THE SONG PLAYED AT THE CONCERT. Are you kidding me?!?! Mavis was entirely unhinged and thinking that the song "belonged" to her and Buddy? I doubt that. Beth knew that was their song and she messed with them. He pretended that Mavis never existed.