Walked on new grout too soon
The tile had already fully set for at least 24 hours before grouting I expect. So walking on it would be safe. The only damage that walking on uncured grout would cause is deforming the grout itself. If you see no evidence of smushed grout either your husband did not step on the actual grout or the grout had hardened enough to not be deformed by his steps. In any case I would not expect any trouble with the grout if it looks fine now.