|"Are hardware and software firewalls supposed to conflict in any way?"|
There is no possible way they can conflict since neither knows about the other.
You would need to ask your friend how they came to this conclusion. Most of us have run both for years with no conflicts.
Heck at work I have two firewalls daisy chained and software firewall engaged on workstations. Pretty well points out your friend came to the wrong conclusion.
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