Why do you want to test it with WhatsApp if the user claims it sends fine?  It just sounds like a coincidence.

Mycelium is an HD wallet so it can generate different addresses for the same HD account, the aim being to use each address only once for privacy reasons.  It may be hard to tell which addresses belong to which account.

If there are several accounts/wallets, they can be shown together or separately.  If the setting in the app is to show them separately (it's an advanced feature), and the selected account is not the one where the address was generated, the app will not show any incoming bitcoins on the Balance or Transactions tabs.  However, regardless of this setting, all accounts and their balances are listed on the Accounts tab.

Has your transaction been confirmed?

There may be various explanations why your party doesn't see your bitcoins, the least likely of which is a malfunction in the Mycelium wallet.  More information is needed to investigate this; at least, the address you sent the money to, to begin with.  If you don't feel comfortable sharing it here, perhaps consider writing to Mycelium support.