Thank you for contacting Ask Extensions.  Mandeville leaves can turn brown for a number of reasons, including fungal infections, pests, overwatering, and poor drainage.

From the picture it does not look like the leaves had any lesions or dark spots, if they did this would indicate a disease.  The pictures don't appear to indicate insect damage.  If your inspection indicates either of these conditions, you can treat the disease with a fungicide and the insects with either hand picking, washing off with soapy water, or using an insecticide.

That leaves the most likely explanation overwatering, and poor drainage. Mandeville does best in full sun with moist but well drained soil.  They do not like to be wet or sit in water.  Pull the plant out of the pot and check the roots.  Healthy roots are firm and white or creamy.  Unhealthy roots will look yellowed or brown and be mushy. If the roots are not healthy wash them off in warm water and cut away any damaged roots. Repot the plant in fresh well draining soil, if using the same pot disinfect before replanting.  Cut the plant back significantly, leaving only a few inches of leaves.

 If most of the plant's roots and leaves are visibly rotted, it might not be possible to save the plant.