Journey Plan 請幫助計劃一個旅遊路線圖[in english]

Actually, it's not difficult to plan the trip, but you didn't mention whether the fashion show is included in the trip or she will spend 2 nights more after attending the show. So I believe the fashion show is included in the trip.
She can stay in Hotel IBIS Gare du Nord, which will be around 100 euro per night.
Day 1:

Leaving London at 9am in the morning. She will arrive Paris at 1pm. The Eurostar ticket will be around 150 pounds round trip.
After settling down in the hotel, she can visit Le Cathedrale de Notre Dame and St. Chapelle by metro. The enterence fee of St. Chapelle is around 8 euro and Le Cathedrale de Notre Dame is free of charge. She can stay there until 5pm.
Then she can visit La Tour Eiffel by metro also. Enterence fee will be 11 euro. Since there will be many people queuing under the tower. Usually you need to take 3 hours to visit it. She can stay there until 8pm
After climbing up the tower, it's the time to back hotel by metro. There is many small resturant near Gare du Nord. She can have dinner there.
Day 2

She can visit Le Basilique de Sacre Coeur and Montmarte in the morning, by metro. Enterence fee is free !! She can stay there until 12pm
She can have lunch in Montmarte. She can stay there until 1pm
Fashion show usually start in the late afternoon. Show will be finished around 8pm. The venue should be easily access by metro as well.
After the show, she can go to Avenue des Champs Elysees to have dinner and have a walk to relax. Also going there by metro.
Then back hotel...
Day 3

She can visit Le musee du Louvre in the morning by metro. Enterence fee is 8 euro and she can stay there until 12pm.
After that, she can go to Department stores, e.g. Au Printemps and Galaries Lafayette to have a look by metro. She can stay until 5pm there.
Then back to hotel to pick up the baggage by metro
Go to Gare du Nord to take Eurostar back to London.


Cost of metro ticket: 10.80 euro for 10 tickets
Cost of meals: 3 days 2 nights around 100 euro
Cost of hotel: 2 night around 200 euro
Cost of enterence fee: 40 euro
Other spending: 50 euro
Cost of train ticket: 150 pounds
Total cost: 400 euro + 150 pounds
The rate of euro to pounds is " 1: 0.671 "
So the total cost will be around 620 euro or 420 pounds