Without a very solid diet there is NO WAY she will lose 50lbs in a tad under 4 months. Especially not with the work capacity she has. I've actually seen people like that GAIN weigth. Why? Because they have a eating problem (you don't get that obese and out of shape if you don't) and a bad relationship with food. They are naturally looking for ways to justify not sticking to a diet. And these people often use training to justify eating more or eating crap. THEY OVERESTIMATE HOW MUCH CALORIES THEY BURN DURING TRAINING AND UNDERESTIMATE THE AMOUNT OF CALORIES IN FOOD.
I'll be honest with you, I HATE planning diets... HATE IT with a vengeance. I'd rather do burpees than write someone a diet. BUT there is NO WAY I would take on a client that has such a goal without being able to control her diet. Anyway, from experience these people are trouble and simply can't stick through the process.
Let's be honest. Someone who has been lazy all their life, have never trained never played sport, never worked hard physically and ate crap since they were a child will not be able to turn things around completely in 4 months. They CAN'T SUFFER... they have no discipline (I may sound harsh but it's the truth).
Someone who suddenly decide to train and eat well for something as trivial as someone else's wedding is not someone I personally want to work with. I've done it in the past when I had no choice (needed the paycheck) and the success rate of these people is less than 25% THEN THEY BLAME YOU FOR THEIR FAILURE. Nevermind that they couldn't stick to their diet or train at a level necessary to make significant changes. Really these people are subconsciously looking for a miracle solution OR someone to put the blame on "yeah Nancy, I tried to lose 50lbs for the wedding, I was doing great but when I hired this trainer I just stopped progressing... he totally screwed me over, taking my money".
That having been said I HAVE worked with obese clients. But those I decided to work with and who had success were committed to be in this for the long run, to change their life/habits, not to get in shape ASAP for one specific occasion.