How dangerous is it to share my iTunes ID?
The danger is considerably high.

Sharing your iTunes ID with another person enables them to make media purchases from any of Apple's stores using the credit card information associated with the account. This includes, but is not limited to:


iTunes Music Store
iTunes Movie and TV Store
iOS App Store
OS X App Store
Apple's online store http://store.apple.com/


The would also be able to access your iTunes ID details and change billing information, address information, view purchase history, sign you up for any Apple-based subscription services like iCloud Storage or iTunes Match, etc.

It also allows the user access to your iCloud account and any email, documents, music, applications, purchases or address book entries you have stored there.

iCloud access could also be used, via Find my Mac/iPhone/iPad, to track your whereabouts if you've enabled the Find My services on any of your devices. They can also lock or permanently wipe the data from these devices, remotely, using the Find My services.

My advice to you is to tell your acquaintance to buy the tracks with their own money since they are, legally speaking, bound to you and your iTunes account by the terms under which you purchased them.