My husband is in middle of getting a crown fitted. Last week he felt ill and now after he came down with a high fever we found that his gums/tongue/mouth is infected.

Is this normal?
Can this be caused by the hygiene of the dentist.

The dentist claims -- trust him -- and take the antibiotic and it will be fine but were nervous about going ahead with the crown after this happened -- does anyone have any experience with this?

Oral infections are not uncommon. You do need to take the antibiotics to kill the infection and get the crown finished.

Yes, it could have been caused by the dentist but confirming or proving it would be difficult if not impossible.

I suggest if you like him - and he's reputable - stick with him. If you have other reasons for doubting him, then have another dentist finish the job.

Any experience how long it will take for the infection to heal?
Do we have to worry about the infection returning?

Oral infections usually take longer to heal than other typical infections. You'll probably get a 10-15 day prescription.

As for it's returning, I would not worry about it - but schedule the finishing of the crown as soon to after the last day of the prescription as you can to be sure.