Heavy metal poisoning is a slow process so a single night being completely nullified wouldn't kill her or Logan. Not to mention only Laura's claws are metal, not her entire skeleton like Logan. A few minutes with their healing factor reactivated would quickly clear any issues.
Depends on the substance and the dosage. Arsenic poisoning can kill in less than a day, and toxic dosages are usually expressed in micrograms, not kilograms. Given the extremely toxic nature described for adamantium and that it takes the majority of Logan's healing factor to counter it, we should probably compare it with the worst examples.
I suppose it would be a way to help differentiate the characters. Null and Laura spend their first night together. Laura wakes up feeling very ill. After an examination, they figure it out. While other characters prefer staying after a date, Laura would be, "That was fun, but you know I can't stick around/you better get back to your room." It would also add an extra dimension to how much she's sacrificing after the mall incident.
It would be a little sad. I imagine Laura would either be the best cuddler or the type to kick you to the edge of the bed so she can have her space. I haven't decided which, yet. Either way, Laura cuddles would be special, and I don't want to disincentive them.
unless their healing factors work like Deadpool's
That too.
Edit: In keeping with having my cake and eating it too and my
Null's powers are wish based speculation above, we can have Laura decide to spend the night despite the fact that it makes her feel ill, and as he's drifting off to sleep,
Null wishes his powers didn't make her sick and problem solved. Best of both worlds.