Please refer to the quick 6-step process for enabling the right to view other users' leads and opportunities:
1) Go to 'User Settings'
2) Search for the user who needs visibility of another user
3) Select the 'People Manager' tab
4) Hit 'Edit'
5) Select the 'Manage User?' checkbox to enable visibility over the respective user
6) Hit 'Save'
After these changes are completed, you will have visibility over the users' leads and opportunities.
Please note, always consult your system administrator before these changes are made.