Can I split an expense with someone who is not a PhonePe user? 

Yes, you can split an expense with someone who is not a PhonePe user. However, they will not be able to view details of the split expense. We recommend that you invite these contacts to install PhonePe to help them settle expenses and make other payments easily. 

Related questions
How is an expense split?
Where can I find the details of my split expenses?