Answer: the earliest you can arrive at the restaurant is 5:40 pm.
The message of your friend says "before you go back to the hotel" which means, according to your itinerary, after the meeting with the client.
Always according to your itinerary, the meeting with the client is expected to last until 5:15 pm, therefore that is the earliest time at which you can leave towards the restaurant.
The restaurant is "25 minutes from the client's office", therefore, the time of arrival at dinner is 5:15 + 25 minutes.
The time is given in the form hh:mm, therefore you need to add the minutes: 15 + 25 = 40.
Hence, the earliest you can arrive at dinner is 5:40 pm.