Is it permissible to take personal belongings left behind by hotel guests after their departure, such as soap and shampoo bottles and sweets? And if not permissible, is it permissible to donate their value in charity?

There is no harm in taking and benefiting from items left by hotel guests because they do not want them. This applies to what you mentioned, so there is no harm for your friend in taking it, nor for you in benefiting from what he gifted to you from it.