My friend just died and his wife wants to use his phone for herself and transfer her info to his new phone without loosing any of his info. Both phones are backed up

For information about how to request access to a deceased family member's Apple account data, and possibly their equipment, click here --> How to request access to a deceased family member's Apple account - Apple Support

Once she has access to the device, if you have signed into the first ID on a device then fortunately when you sign out of that ID, do not erase the device, then sign in with a different ID, it asks you in most cases if you wish to merge the data from the two IDs on your device (toggle the ones you want to on). Say yes and later delete individual items from the other ID that you do not want. Note, in How to save apps and progress on one Appl… - Apple Community a Community Specialist says, "you will not be able to transfer the meta data from one Apple ID to another. Once you switch to another Apple ID the information associated with the old Apple ID will be gone except for physical content on the device like photos and videos." Also turn off storage optimization before doing this. With Optimize Storage, smaller, space-saving photos and videos are kept on your device while all of your original, full-resolution versions are stored in iCloud. Make sure photo optimization is turned off. Manage your photo and video storage - Apple Support

iPhones are personal devices; you can have only one person’s data on the phone at a time. She can get a 2nd inexpensive phone or iPad for her husband’s data and transfer his data to it, or get a 3rd party app such as TouchCopy or iMazing and transfer the content from his phone to a computer where she can archive it.