A friend of mine utilized IPFS to store data files regarding some user info.But for an android application of the same, he retrieved all hash values(same user but different files) and stored it in a database.I claimed that the base concept of decentralization wasn't achieved by it but he said that it's not the case. Could anyone comment on this?
A quick comment : If your friend uses IPFS to stores files then he is right, he's building a decentrlized application. However, storing the files paths in a centrlized database won't harm its underlying decentralized nature but it might be a botlneck or a SPOF (single point of failure) in case where these paths are lost or corrupted, users won't be able to access the files on IPFS. But even in this later case he will be able to recover back the database.
Contrary to popular belief IPFS doesn't make anything decentralized, per se. The protocol enables decentralization but in reality if you upload files to IPFS it's all centralized: the data resides only in the IPFS nodes to which you upload it to.
In theory any IPFS node can replicate any data they wish but currently there is no incentive to do so. Therefore nobody replicates your data unless you explicitly replicate it yourself to other nodes you control. IPFS has plans/ideas to add incentives (Filecoin) but those plans are somewhere in the far future - unless there has been some recent development which I'm unaware of.
So storing files to an IPFS node you own and storing those hash values to a centralized database has very little to do with decentralization.