Hey there MandyDolldoll,
We understand audio messages are not playing for your friend in the Messages app. They play on your device but do not on theirs. We'll be happy to help.
If this is only happening with one contact and only on their device, it sounds like the issue may be with your friends device. We'd recommend passing along this info to them.
To begin, let's make sure the iPhone is fully up-to-date. Use the steps here: Update your iPhone, iPad, or iPod touch
If it's already up-to-date, let's complete a standard restart with the steps here: Restart your iPhone
Afterwards, let's check storage to make sure there is enough available. Low storage could cause issues. Use these steps: How to check the storage on your iPhone, iPad, and iPod touch
You could also test this with other contacts or even yourself to see if the issue still happens. This would help isolate what's happening.
Let us know how that goes or if you have any questions. Cheers!