What if I put in an oil/gas mixture into my lawnmower by mistake?

My son put in an oil/gas mixture into our lawnmower. I use the mixture for my weed eater. Will this ruin the lawnmower? They just put in the mixture yesterday and mowed the gr. When I asked him to mow a section of gr today and he said the lawnmower won't stay 'on'. He starts it, but when he tries to engage the blade, it turns off. What should I do?