My friend isn't getting the invite to my Calendars

Invite others to an event

• Tap the event, then tap Edit near the top of the screen.
• Tap Invitees.
• Enter the names or email addresses of people you want to invite, or tap. to select Contacts. Note: For you to send a calendar invitation to a contact, they must have an email address. ... 
• When you're finished, tap Done.

Send invitations in Calendar on iPhone - Apple Support (IN)

Invited subscribers must have an Apple ID. (Note: not an Apple Device, just an Apple ID)

Make sure that the people you are sending have a valid Apple ID /iCloud address using the email address which you have chosen while inviting them to view your Shared Album. Instead, try using a phone number that has iMessage activated as another option if you do know the Apple ID of the person whom you want to share with.