Yes, you can enable the Advanced digitizing tools whilst editing. When enabled, there is an active function which snaps the selected vertices to the nearest common angle. You can open the advanced tools from the menubar:
View > Panels > Advanced Digitizing Panel
Click the left-most icon to ensure it is enabed and set the common snapping angle:
Then use the Node tool (or Vertex tool if using QGIS 3.0), select the vertices and move the cursor vertically whilst staying close to the 90 degree angle:
You can do the same horizontally:
Note: the layer must be in a projected CRS in order to use the Advanced Digitizing Tool.
Tested on QGIS 3.0 for Win7 64-bit.