I just completed the quest and by chance it was with the help of a Glitchen who for some reason was waiting near a Dirt Pile. It came to light that he was on the same quest as he helped me dig when I started digging next to him so in turn I helped him.
I still haven't managed to do this quest although I'm hopinh to get round to it soon so if you'd like to do an exchange of I help you and then you help me let me know!
From what I've observed, I think it's not so much asking someone to help dig as someone joining in after you've started digging. When I was trying to do it I'd ask someone to dig with me and then they always started digging first so I never got a credit for it... The person you're asking doesn't necessarily realise you're trying to complete this quest rather than just get better rewards from digging by doing it together.