It seems your friend is having sensitivity in his/her teeth
. If it is in a particular tooth
then kindly ask him to get an X-ray
done of the tooth having sensitivity. May be root canal treatment
has to be done followed by a cap/crown. If it is generalized then he will have to get scaling done and use an anti sensitive toothpaste for twice daily for 15 days. Ask him to brush his teeth twice daily using correct brushing technique to maintain his oral hygiene.