"What is so difficult about that?"
You've forgotten some options:
I dunno about your mates, but this is the kind of thing I expect from drunk people:
F) drunk person is awake when OP goes into labour and decides to be a "helpful midwife", except the only person who thinks he is being helpful is him, he is too pissed to understand and just won't leave the OP in peace
G) drunk person is drunk, vomiting and incapable of standing. OP's DH has to deal with this, leaving her to labour alone
H) drunk friend is fast asleep, too drunk to wake up - but labour is progressing and OP and her DP have to go to hospital. What do they do with him? How will he lock up? Should they leave keys? Will the house smell of booze when they bring the new baby home? OP doesn't want to have to think about this stuff, she wants to focus on her labour.
I) drunk friend is only a bit tipsy but too drunk to drive. OP goes into labour while he is still awake. She finds his presence in the house an intrusion. She feels vulnerable and not comfortable to be herself and go with the labour, even though she's in her own home. Consequently contractions slow down, as she's stressed, making for a longer, more difficult labour
... and so on.
Or the mate could just not drink.