To know if activated charcoal could be helpful in this situation, otherwise to know how to help.

While activated charcoal is usually given in rare cases of poisoning under hospital supervision, it is not recommended for your situation. Instead, it is important to prevent dehydration during diarrhea by providing rehydration solutions. If lactose intolerance is the issue, symptoms typically last up to 48 hours. There are various products available at pharmacies to help improve lactose digestion if your partner wants to consume lactose-containing products in the future.